Diode detectors for RF measurement Part 1: Rectifier circuits, theory and calculation procedures.

This article addresses the subject of RF signal detection from the point of view of those who design and calibrate impedance matching bridges and other measuring instruments operating in the HF and VHF radio ranges. The principal discussion relates to the simple diode peak-detector; and covers the different possible circuit configurations, the associated theory, and the numerical methods needed for data analysis. Circuit techniques used to linearise the diode detector output are discussed in a separate document (Part 2). Analysis of the detector transfer characteristic for sinusoidal input shows that the AC-induced error term involves the zero-order modified Bessel function of the first kind (I 0 ). This result is not new, but it is often disregarded. The dynamic contribution is quite unlike the error that occurs for DC input, regardless of any compensatory modification of circuit parameters; which means that linearity correction schemes using a reference diode to produce a DC amplifier with a complementary gain law can never be perfect. It is also shown however, that the AC error is independent of frequency provided that the smoothing capacitor is 'large'. This means that the tracking detector system, which involves automatic self-calibration against a low-frequency precision rectifier, is theoretically sound. By considering the power dissipated in the detector, it is shown that the input impedance can be calculated using first and zero-order modified Bessel functions of the first kind (I 0 and I 1 ). This allows the determination of detector transfer-functions that take source impedance into account. When this facility is combined with the ability to calculate the dynamic component of the peak detection error, a measurement of DC output taken with a calibrated voltmeter can be converted into a measurement of AC input without the need for an AC reference. The computation procedures required are not simple, but they are described in detail and given as Basic algorithms readily adaptable to any programming environment.